Previous studies have demonstrated the presence of antibodies to novel halothane-induced liver neoantigens in sera from patients with halothane hepatitis, and have suggested that these neoantigens may play an immunopathological role in development of the patients liver damage. The antibodies have also been shown by immunoblotting to recognize several distinct halothane-induced liver polypeptide neoantigen fractions (100kDa, 76kDa, 59kDa, 57kDa, 54kDa) in liver microsomes that have been covalently modified by the reactive trifluoroacetyl halide (CF33COX) metabolite of halothane. In this paper, the trifluoracetylated (TFA) 59 kDa neoantigen (59 kDa-TFA) was purified from liver microsomes of halothane treated rats by immunoaffinity chromatography with anti-TFA IgG. Antibodies were raised against the 59 kDa-TFA protein and were used to purify the native protein from liver microsomes of untreated rats. Based upon its apparent monomeric molecular weight, NH2-terminal amino acid sequence, catalytic activity, and other physical properties, the protein has been identified as a previously characterized microsomal carboxylesterase (EC 3.1.1.1) and not a novel form of cytochrome P-450 as suggested in last years annual report. A similar affinity chromatography approach may be used to purify and characterize the other halothane-induced TFA neoantigens. Immunization of animals with the purified neoantigens, prior to the administration of halothane, will provide an experimental approach for the development of an animal model for this drug hypersensitivity and similar approaches may elucidate other drug hypersensitivities.